Study On The Mechanism Of Activated Cadmium In Iron Oxide By Biochar Regulating Dissolved Organic Matter

Iron oxide is one of the important minerals and active substances in the soil sphere.It exists widely in nature.Due to its large specific surface area and relatively stable chemical properties,iron oxide can adsorb and fix heavy metals and has a profound influence on the migration and transformation of heavy metals in the soil.Biochar is widely used as passivating agent to repair polluted farmland soil because of its excellent physical and chemical properties,strong adsorption capacity for heavy metals and environmental friendliness.Some components of the soil,such as dissolved organic matter(DOM),are affected by the application of biochar,which affects the iron oxides and causes the Cd fixed in the iron oxides to be released for redistribution.Therefore,clarifying the effects of biochar on the composition and properties of soil DOM and the activation mechanism of Cd in iron oxides is helpful to further reveal the microscopic mechanism of biochar’s activation of iron-oxide binding Cd in alkaline soil.In this study,several kinds of iron oxides and iron oxides containing different forms of Cd were prepared by laboratory synthesis method,and wheat straw biochar modified with different pyrolysis temperatures and phytic acid and peat soil soluble organic matter(PDOM)were selected.The activation of Cd in iron oxides by hydration conditions,the activation mechanism of Cd in iron oxides by biochar dissolved organic matter(BDOM),and the activation mechanism of Cd in iron oxides by biochar regulation of PDOM were studied in order to explore the micro-mechanism of biochar activation of iron-oxide bound Cd in alkaline soil.To provide a solution for precise remediation of alkaline soil contaminated by Cd.Through the above research,the main conclusions of this paper are as follows:(1)Six kinds of iron oxides containing different forms of Cd were synthesized and the factors of Cd activation were analyzed.In the process of synthesizing isomorphous substitution of iron oxides containing Cd,the coprecipitation of Cd and Fe will lead to the weakening of XRD peaks,the increase of half-peak width,the low angular displacement of peaks,the increase of the corresponding mesh spacing d,and the change of the local substructure of iron oxides.The content of isomorphous substitution Cd in the synthesized goethite,ferrihydrite and hematite is 121.23 mg/g,91.76 mg/g and 9.6 mg/g,respectively.In the process of adsorption of CD-containing iron oxide on synthetic surface,the adsorption of Cd on iron oxide is more consistent with the quasi second-order kinetic model and Langmuir adsorption model,and the adsorption rate of Cd on hematite is faster than that of goethite and ferrihydrite.The maximum adsorption capacity of Cd on goethite,hematite and ferrihydrite are 31.19 mg/g,21.85 mg/g and 20.05 mg/g,respectively.In the process of activating Cd in iron oxides,the activation rate of Cd in the surface adsorption of iron oxides containing Cd in different hydration conditions is greater than that in the isomorphous substitution of iron oxides containing Cd.Different pH values,different types of cations and concentrations and DOM can promote the activation of Cd in iron oxides,and low pH value,Ca2+ and PDOM have stronger activation effects.(2)In order to further explore the activation effect of DOM on Cd in iron oxides,biochar at different pyrolysis temperatures was selected to extract BDOM,and the activation mechanism of different BDOM on Cd in iron oxides was analyzed.In the process of activating Cd in iron oxide,the activation rate of Cd in the adsorption of iron oxide containing Cd on the surface of BDOM is greater than that in the isomorphic substitution of iron oxide containing Cd.Among them,dissolved organic matter(WBDOM3)in biochar pyrolysis at 300℃ and dissolved organic matter(P-WBDOM7)in biochar co-pyrolysis with phytic acid at 700℃have the best activation effect on Cd in goethite.The activation rates of Cd were 70.25%and 90.35%,respectively,in the surface adsorption of Cd-containing goethite(Cd-Gt),and 19.13%and 22.13%,respectively,in the isomorphous substitution of Cd-containing goethite(GtCd).Among the different components of WBDOM3,the fraction<0.5kDa accounted for the smallest(7.41%),and the activation rate of Cd in iron oxide was the largest(22.09%and 6.69%for Cd-Gt and GtCd,respectively).Carboxyl carbon is the key component of Cd activation in iron oxides.It may promote Cd release in iron oxides through complexation with Cd or ligand dissolution with iron oxides.In PWBDOM7,the 1kDa-0.5kDa component has the smallest proportion of 11.99%,and the activation rate of Cd in iron oxide is the largest,which is 24.61%in Cd-Gt and 10.81%in GtCd.Hydroxyl group is the key component in activating Cd in iron oxides.It may promote Cd release in iron oxides through complexation with Cd or ligand dissolution with iron oxides.(3)The key components of Cd in activating iron oxides in PDOM were further explored,and the activation effect of PDOM on Cd in iron oxides under the control of biochar frameworks(WB3,P-WB7)during pyrolysis at 300℃ and 700℃ was analyzed.In the process of activating Cd in iron oxides,the<1kDa components of PDOM play a dominant role.The key component is methoxy-carbon,which may promote the release of Cd in iron oxides through complexation with Cd or ligand-promoting dissolution with iron oxides.The biochar framework can affect the content and composition of PDOM,and the influence of P-WB7 on PDOM is greater than that of WB3,and the<1kDa component of PDOM is the most affected.After WB3 and P-WB7 regulation for 1 day,15 days and 30 days,the activation rate of Cd in iron oxides of each component of PDOM(WB3P1,WB3P15,WB3P30,P-WB7P1,P-WB7P15,PWB7P30)decreased significantly.Among them,the activation rate of Cd in iron oxides of<1kDa components changed significantly with the increase of regulated time,and decreased by 20.77%at most.It is further proved that WB3 mainly adsorbs alkoxy carbon in<1kDa in PDOM through infrared,nuclear magnetic and other technological means,and releases carboxyl carbon with the increase of time.As a result,the activation rates of<1kDa components in WB3P1,WB3P15 and WB3P30 to Cd in iron oxides showed a trend of first decreasing and then increasing with the increase of action time.P-WB3 mainly adsorbed the alkyl carbon in<1kDa in PDOM,and released the-PO4 group with the increase of time.Therefore,the activation rate of<1kDa components in P-WB7P1,P-WB7P15 and P-WB7P30 to Cd in iron oxides increased with the increase of time.